This invention relates broadly to the field of single turn potentiometers, of the type commonly referred to as "trimmer" potentiometers. More specifically, this invention relates to the construction of a fluid-tight seal between the rotor of such a potentiometer and the housing in which the rotor is installed.
Single turn "trimmer" potentiometers are well-known in the electronic field, and have long been useful in a wide variety of applications. The typical trimming potentiometer includes an insulating housing (usually plastic) encapsulating an insulating substrate (usually a ceramic), on which is formed a resistive element and several metallized conductive elements to which the leads are attached. A rotor is installed in the housing for rotation therein, the rotor having an interior surface on which is mounted a wiper contact for establishing electrical contact between the resistive element and one of the metallized conductive elements.
In many applications, it is necessary or desirable to seal the interior of the potentiometer from the entry of particulate or liquid contaminants, particularly cleaning solutions. Typically, this is done with an O-ring placed between the rotor and the housing. See, for example, U.S. Pat. No. 4,626,823 to Smith. There are a number of drawbacks, however, to the use of O-rings. For example, O-rings tend to deteriorate with age, thereby resulting in a degraded seal. Also, the resiliency of O-rings gives them a tendency to "wind up" when the rotor is turned, and then to spring back, thereby shifting the setting of the rotor. Moreover, the resiliency of the O-ring tends to decrease with time, thereby altering the rotational torque characteristics of the rotor. Finally, with respect to miniature potentiometers (i.e., those with horizontal dimensions of approximately 5 mm or less), it is very difficult to obtain precision O-rings sufficiently small in size to fit such components.
Consequently, the prior art has sought alternatives to O-rings to perform the necessary sealing function. For example, U.S. Pat. No. 3,518,604 to Beaver et al. shows a potentiometer with a rotor that is flared outwardly at its periphery to seal against the edge of the aperture in which the rotor is seated. U.S. Pat. No. 3,531,860 to Paine et al. shows a potentiometer in which the rotor has a peripheral flange with an upturned lip that seals against an internal shoulder in the housing. While these prior art approaches may yield satisfactory results in some applications, there has been a continuing need for improving the integrity and durability of the rotor-to-housing seal, especially in potentiometers of so-called "hot rotor" type, in which the rotor is made of a metal. In this type of potentiometer, a fluid-tight seal must be formed between the metal rotor and the plastic housing, and prior art techniques have heretofore yielded less than satisfactory results.